1. Field of the Invention
This invention pertains to the general field of loudspeaker systems for use in motor vehicles. In particular, it provides a new system for improving the quality of sound reproduced in the vehicle, especially in the passenger cabin of trucks.
2. Description of the Prior Art
With the continuously improving quality of high fidelity sound in general, people strive to adapt the characteristics of home stereo systems to the driving environment of their motor vehicles. Unfortunately, limited space and poor noise insulation in such vehicles render this task rather difficult and sometimes cumbersome.
With respect to sound quality, a greatly limiting factor in a vehicle is the unavailability of sufficient space to accommodate the appropriate equipment. Obviously, the most important factor in the system is the technical quality of the speakers used, but even the best quality speakers are large and require much space for installation in the passenger compartment of motor vehicles. Sound systems often separate the emission of low frequency ("bass") from midrange and high frequency waves by channeling them to separate speakers, normally referred to as "woofers." They tend to be very large because the quality of their performance is related, among other things, to the size of the diaphragm and vibrating cone in the device. While this feature does not constitute an inherent problem for home and other uses, it does for motor vehicle applications, where the space available is generally very limited. This is particularly true in the case of trucks because passenger cabin space is often sacrificed in order to maximize the length of the truck bed and, therefore, the utility of the vehicle itself.
To circumvent these unavoidable problems, motorvehicle and stereo systems manufacturers have developed amplifiers that enhance the low frequency sounds by manipulating the relative output power of each frequency range to suit the listener's expectation. They have also taken advantage of opportunities to improve sound quality by exploiting the enhancing effect of baffling materials and of vehicle geometry in the reproduction of low audio frequencies.
For example, by exploiting the ability of acoustic baffles to radiate sound with a lower cutoff frequency, U.S. Pat. No. 4,136,755 to Goes describes a speaker system that is incorporated into a hollow support structure filled with sound absorbing material, such as foam plastic. Because this support structure can perform other functions in addition to housing the speakers, such as providing a frame for dashboard instrumentation or constituting a part of a passive restraining system, the space occupied by this speaker system is not dedicated exclusively to sound reproduction. Therefore, the woofer can be larger than could otherwise be afforded in the average size automobile. Moreover, because of the acoustic characteristics of the absorbing material, bass performance is improved. This method of low frequency sound enhancement, though, relies on the baffling effect of the sound absorbing material, rather than on the true reproduction characteristics of the speaker itself.
Similarly, U.S. Pat. No. 4,567,959 to Prophit discloses a low frequency speaker designed to take advantage of truck cabin geometry to enhance the sound produced by a relatively small apparatus. Because of its size, the device can be installed behind each seat in the cabin without excessive sacrifice of useful space. On the other hand, it requires positioning the speaker within a few inches of a three surface corner to optimize its functioning, thus limiting space utilization in the vehicle.
It is the purpose of this invention to address the problem presented by space limitations in designing a speaker system with high fidelity in the low frequency range for motor vehicle passenger compartments. The invention is directed especially to trucks, where the interior noise in the cabin is normally relatively high and the available space is particularly small. A solution is found in utilizing the space behind the rear windshield of the vehicle to install speaker systems containing full size woofers of standard specifications.